The present invention relates to a novel propylene polymer composition useful for production of moldings.
Isotactic polypropylene is used in a number of industrial fields because it is excellent in properties such as stiffness, heat resistance and surface luster and further is inexpensive. Isotactic polypropylene, however, is limited in its use because it is poor in impact strength and cannot be used in applications in which high mechanical impact is applied.
As methods for improving impact resistance of polypropylene, for example, (1) a method in which polypropylene and polyethylene or an ethylene-propylene elastomer are mechanically blended, and (2) a method for an impact resistant polypropylene composition usually called "block grade" in which propylene is first polymerized at the first step and at the second step, propylene and ethylene are copolymerized, are known.
Particularly, in the method (2), compositions prepared by uniformly mixing the homopolypropylene obtained at the first step with propylene-ethylene copolymers having various ethylene contents in various ratios have been proposed (Japanese Patent Publication No. 1836/1964)
These compositions, however, have a disadvantage in that excellent surface luster characteristic of polypropylene is reduced, although they are all improved in impact resistance. In recent years, with an extension of applications of impact resistant polypropylene, characteristics required for the polypropylene have been changing. That is, not only impact strength and stiffness, but also appearance characteristics, e.g., surface luster of molding, have becoming more important.
In recent years, a method in which at the first step, a small amount of ethylene is copolymerized for the purpose of improving physical properties of impact resistant polypropylene composition has been employed. In accordance with the above method, propylene and ethylene are copolymerized at the first step so that the ethylene content is 0.1 to 10% by weight, and the copolymer thus obtained is mixed with a propylene-ethylene copolymer obtained at the second step so that the ethylene content is not less than 70% by weight (see Japanese Patent Application Laid-Open No. 25291/1972). The method is intended to improve weld strength at the time of molding. However, such high impact strength that is expected as a block copolymer cannot be obtained Furthermore the method has a disadvantage of decreasing stiffness of molding.